HIV infection results in widespread immune activation and dysfunction, extending to both the acquired and innate immune system. Modulation of Natural Killer (NK) cell function has been described during HIV infection, and recently we have found that NK cells express CD4 on their surface in vivo. To better understand the role of CD4+ NK cells in HIV pathogenesis, we have established an in vitro culture system using highly purified NK cells which results in up to 50% CD4 expression by NK cells. This has allowed us to have sufficient quantities of cells to assess the function of CD4+ NK cells, the function of CD4 on NK cells, and effects of HIV infection on NK cells. As CD4 expression causes NK cells to be susceptible to HIV infection, and these cells could potentially function as a reservoir for HIV, we believe it is crucial to further study CD4 expression on NK cells to enhance our understanding of HIV pathogenesis. This proposal describes studies which will enable us to learn more about CD4 expression and function on NK cells and the role of NK cells in HIV pathogenesis. Our Specific Aims are to: 1) Determine the function of CD4+ NK cells in terms of their ability to mediate cytotoxicity and produce cytokines and chemokines; 2) Determine the consequences of CD4 ligation on NK cells using a combination of functional bioassays and gene expression based technology; and 3) Determine the effect of HIV infection on NK cell function, for this Aim both wild type and reporter viruses will be utilized allowing the purification of live, HIV infected NK cells for further study. The experiments outlined in this proposal will help us to better understand the regulation of CD4 on NK cells, the function of this molecule on a "nontraditional" cell type, and the consequences of HIV infection on CD4 expression and the function of CD4+ NK cells. We hope these studies will better define HIV pathogenesis related to NK cells, and will also allow us to determine the possible effects that the appearance or loss of this subset may have on HIV disease progression, HIV reservoirs, and antiviral immunity.